Chaff

Chaff is a cloud of tiny metallic or metal-coated fibers dispensed from an aircraft or ship to confuse radar. Each strip is cut to roughly half the wavelength of the threat radar, which makes it resonate and throw back a strong echo, so a handful of grams blooms into a radar return the size of an airliner. The defending radar or the incoming missile sees a sudden blob of false targets and, if the timing is right, chases the cloud while the real aircraft turns away.

An old trick that refuses to retire

Chaff dates to 1943, when British and German scientists both independently discovered that dropping foil strips blinded radar, and the British used it, codename Window, to devastate German air defenses over Hamburg. Eighty years later it still rides in the countermeasure dispensers of nearly every military aircraft, because the underlying physics never stopped working. It is cheap, passive, drawing no power and emitting nothing to home on, and instantaneous, a whole cloud appears in a fraction of a second.

Modern radars fight back, and the contest defines chaff's current role. Doppler radar filters by velocity, and chaff, being light, decelerates to the speed of the wind almost immediately, so a pulse-Doppler system rejects the slow-moving cloud while tracking the fast-moving aircraft. This is why chaff alone rarely defeats a modern missile and why it is used as one instrument in a defensive suite, combined with a hard maneuver into the cloud, with electronic jamming, and with flares against infrared threats. Deployed in the right geometry at the right instant, still against the right radar, it remains genuinely useful, which is why nobody removes it.

The naval version scales up. Ships fire chaff rockets that burst into enormous clouds hundreds of meters away, seducing anti-ship missiles off the hull, part of a soft-kill layer that sits alongside hard-kill guns and interceptors.

How is chaff different from a flare?

Different threat, different physics. Chaff decoys radar-guided weapons by mimicking a radar return. Flares decoy infrared-guided weapons by burning hotter than the aircraft's engines to pull a heat-seeking missile off the exhaust. Aircraft carry both because they cannot know at the instant of launch which kind of seeker is chasing them, and the combined dispenser firing both on a preset or automatic program is standard fit from fighters to transports.